This invention relates to an electron microscope and a specimen stage positioning control method for the electron microscope suitably used for inspection and evaluation of a semiconductor in the semiconductor fabrication field.
With the recent micronization of semiconductor elements, the inspection and evaluation apparatuses as well as the fabrication apparatus thereof have been required to be correspondingly higher in accuracy. Normally, in order to evaluate whether the shape and size of a pattern formed on a semiconductor wafer are correct or not, a scanning electoron microscope having a critical dimension measuring function (hereinafter referred to as the critical dimension measuring SEM) is used.
In the critical dimension measuring SEM, an electron beam is radiated on a wafer and the secondary electron signal obtained is subjected to image processing, and from the change in contrast thereof, a pattern edge is discriminated thereby to determine the size (see, for example, JP-A-9-166428, paragraphs “0012” to “0016”, FIG. 4).
For this reason, in order to keep up with the design rule of 35 nm node, it has become a critical subject to obtain a secondary electron image of fewer noises in the magnification of not less than 300 thousands for observation. Also, in order to improve the contrast by superposing a number of images one on another, the vibration and drift of nm order of the stage on which the wafer is mounted and held are required to be suppressed.
In the critical dimension measuring SEM, the wafer evaluated is returned to the fabrication line, and therefore, the wafer cannot be split into small segments. The evaluation of the whole wafer surface, therefore, requires a wafer stage having a sufficient stroke to make the evaluation possible.
Presently, the main size of the wafer employed is 300 nm, which considerably increases the size of the stage used with it. At the same time, from the viewpoint of improving the throughput, a high output drive mechanism for increasing the stage speed is required, which in turn generates heat in the drive shaft of the motor, resulting in a temperature increase.
As a conventional means for avoiding the table drift due to thermal expansion and compression, what is called the servo control technique is available in which the stage position is monitored in real time and the result of monitoring the table behavior is fed back to determine the motor control amount and the electron beam deflection amount (see, for example, JP-A-2002-126964, paragraphs “0005” to “0016”)
Controlling the position of the stage and the electron beam on the nm order, however, requires a system for measuring the stage position with high resolution at high speed, a servo circuit and a servo motor for performing the servo control operation and the adjustment thereof, thereby leading to a system complication and an increased apparatus cost. Also, in the servo control operation, a minor vibration is generated even in stationary state, which may vibrate the whole apparatus and affect the secondary electron image.
To cope with this problem, a technique has been conceived in which a gap of 20 μm to 100 μm is formed between the movable table and the drive shaft which is separated while in stationary state (see, for example, JP-A-2004-134155, paragraphs “0007” to “0011”). According to this technique, the control operation by an open loop using a pulse motor is possible.
According to the technique disclosed in JP-A-2004-134155 employing the structure having a gap in the drive unit, however, a dead zone like the backlash of a gear occurs and a smooth fine feed becomes difficult. In the case where the movement over a distance shorter than the gap, therefore, the gap is sided in one direction by provisional movement over a long distance, after which the control operation is performed for fine feed toward a target position. As a result, a longer time is required to reach the target position, thereby giving rise to a new problem of a reduced throughput.